Wow. What an incredibly hurtful thing to do. I agree with other comments, you need to go right to the source and just ask your neice outright. If she says he's not invited, ask why. My opinion is that your loyalty is to your son and if they choose to exclude him, then I'd make it very clear that none of you will be attending. It's a slap in the face to all of you and I wouldn't tolerate it for the sake of peace. Your son deserves better.